The invention relates to a manipulator for controlling the position of what is referred to as an "end effector"; which term covers such things as tools, gripping devices and holding devices. The invention concerns the type of manipulator which includes at least three arms each connecting the end effector to a fixed point on a supporting structure. In such a construction the effective length of each arm is adjusted to position the end effector as required.
One known manipulator of the aforementioned type is described in our U.K. Patent Specification No. 2083795. This system is able to manipulate an end effector to any desired point in a volume of space and also to orientate it at a desired orientation at that point. For this purpose the manipulator includes six, instead of three, control arms. The problem with this arrangement is that it is relatively complex and that the degree of adjustment of orientation of the end effector, for any particular position of it in the aforementioned volume of space, is limited.
Another known manipulator is described with reference to FIG. 1 in U.S. Pat. No. 4,407,625. This comprises three arms connected at their bottom ends by a pivot mechanism allowing all three to pivot relative to each other about a common point. The end effector is connected to one of the three arms. A manipulator of this type does not allow orientation of the end effector when it has been moved to a desired point. Such adjustment of orientation is not necessarily a disadvantage since the end effector itself can be provided with a special wrist mechanism providing a freedom for adjustment of orientation through large angles (greater than the angles provided for by the arrangement of U.K. Specification No. 2083795). Also of course some types of end effector, including that shown in U.S. Pat. No. 4,407,625, do not require such adjustment of orientation. A problem however with the arrangement described in the U.S. Specification is that the pivot arrangement at the bottom of arms is relatively complex and that there is no way in which the end effector can be connected directly to the pivot point. This means that, to move the end effector to a given point, it is necessary to take into consideration an offset between the end effector and the pivot point. The need for this complicates the control system.